Recently, obtaining individual optical lens by producing a wafer-shaped plate member (a wafer lens) in which multiple optical lenses are formed and then dividing into single pieces has been studied. As a method for transferring micro-optical parts on a wafer scale, producing a first generation reproduction tool made of, for example, resin by transferring with repeated use of a small master die, then, producing a plurality of sub-master dies from the first generation reproduction tool, and producing a plurality of second generation reproduction tools provided with multiple micro-optical elements from the sub-master dies has been proposed (see Patent Literature 1). The wafer-shaped first generation reproduction tool obtained by this method is a molding die for producing a subsequent molded product and is a tool constituted by a resin-made shape transfer layer formed on a substrate.
In addition, as a method for forming a molding die, which is used for producing a wafer lens and in which a resin-made shape transfer layer is provided on a substrate, in order to, for example, prevent unsuccessful mold release at the time of releasing a molded product from a master substrate, a method for producing a molding die by forming a plurality of recessed portions which are closed inside them on a substrate for molding die, injecting a resin material into each of the recessed portions, and then pressing the recessed portions with a master die has also been proposed (see Patent Literature 2).
Recently, versatility of small-sized optical lenses has been increased and it has been required that the optical lenses have intended accurate lens shape so that desired optical performance may be demonstrated. In order to improve the optical performance, a plurality of optical lenses may be stacked. From these viewpoints, it is required that the thickness of a resin layer of the wafer lens is not excessively large. If the thickness of the resin layer of the wafer lens is excessively thick, it is possible that desired optical performance is not demonstrated or that warpage, deformation and the like may be caused in the wafer lens due to increased stress of the resin layer. Further, there is a possibility that the entire size is increased when the optical lenses are stacked. There is also a problem that the material cost may be increased and the curing time may become longer.
In order to prevent the thickness of the resin layer of the wafer lens from being excessively large, it is required to produce the molding die which includes the resin-made shape transfer layer described above in consideration of this, and it is necessary that molding is performed with the master die being brought close to the substrate for molding die as much as possible at the time of producing the molding die. This is because, if the resin-made shape transfer layer of the molding die becomes thick, it is not possible to reduce the thickness of the resin layer of the finally obtained wafer lens since the shape is transferred also to a molded product molded using this molding die.
Typically, it is necessary to press the master die against the substrate for molding die with large pressure to bring the master die close to the surface of the substrate for molding die in a state in which a resin material is disposed between the master die and the substrate for molding die. Therefore, the size of a production device becomes large and it becomes difficult to ensure the positioning accuracy of the master die. Further, if the master die is inclined for some reason, there is also a possibility that the master die may be in contact with a sub-master substrate, thereby damaging the sub-master substrate and the master die. It is also considered that the resin material overflows from a periphery of the master die at the time of molding and the overflowed portion forms an unintended shape. Especially if recessed portions which are closed inside them are provided on a substrate for molding die as in Patent Literature 2, the space between the peripheral edges of the recessed portions and a peripheral edge of the master die is significantly narrow at the time of molding and, therefore, a possibility that resin overflows is even more increased due to, for example, variation in the resin amount injected in the recessed portions and minor errors in distance between the master die and the substrate for molding die. If the distance among each molding position by the master die is shortened in order to increase the number of optical lenses to be obtained from a single wafer lens, the overflowed resin may gather and rise, thereby forming projections. Therefore, a possibility of producing an unintended shape is even further increased. On the other hand, if the resin amount is reduced so that the resin material does not overflow from the periphery of the master die during the molding, a step portion is formed at the periphery of the master die which may become an overhang shape or an undercut shape (a projecting shape) having a downward slope. It has been found that such a step portion creates mold release resistance in the next molding process in which this molding die is used. The mold release resistance causes local shape distortion, failure in molding, such as breakage of the step portion, a decrease in molding durability of the substrate, and the like. Especially in a case where the total of edge sides (end sides) corresponding to the circumference of the master die is long, if, for example, the piece is large in size with an increased number of optical surfaces inside the master die, the mold release resistance generated locally increases and various harmful effects become significant.